(Stroke. 1998;29:2211-2213.)
© 1998 American Heart Association, Inc.
Evaluation of Carotid Artery Stenosis by Power Doppler Imaging
Christian Arning, MD
Department of Neurology,
Barmbek Hospital,
Hamburg, Germany
To the Editor:
Steinke and coworkers1 recently reported on the possible
advantages of power Doppler imaging (PDI) for quantification of
stenoses of the internal carotid artery (ICA): similar to
angiography, it should be possible to assess the degree of
stenosis of the ICA as the percentage of the diameter reduction
from the longitudinal image.
We have tested this examination procedure by comparison with the usual,
validated sonography criteria on 40 cases of ICA stenosis.
Included in the observation period of the study were all consecutive
stenosis findings for which color-coded duplex sonography
(CCDS) fulfilled the following criteria: (1) detection of a local flow
acceleration in the ICA, (2) peak flow velocity of 
1 m/s (measurement
of the jet flow at the stenosis maximum or, in cases of sound
extinction there, directly distal from it), and (3) detection of flow
disturbances. Quantification was achieved using the known
Doppler criteria.2
In 22% of the cases an adequate evaluation by PDI was not possible: in
6 stenoses the residual lumen could not be demonstrated because
of sound extinction; in an additional 3 stenoses, the vessel
wall could not be imaged adequately for determination of the degree of
stenosis. In the remaining cases the degree of stenosis
was regularly underestimated in comparison with the Doppler
criteria (Figure 1
).
Figure 2
illustrates this for the example of a high-grade stenosis of
the ICA: the stenosis is very poorly demonstrated in the PDI
(panel A). Angiography and conventional CCDS findings with
determination of the peak systolic velocity were in agreement,
and both revealed the high-grade stenosis (panels B and C).
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Figure 2. Comparison of PDI, Doppler criteria, and
angiography for a high-grade stenosis of the ICA (sonography
was performed before and after angiography and revealed identical
results). A, Power Doppler mode. B, CCDS in velocity mode with
determination of the peak systolic velocity. C, angiography
(selective digital subtraction angiography). (We are grateful to Prof
Dr H. Zeumer, head of the Department of
Neuroradiology, University Hospital
Hamburg-Eppendorf, for kindly providing the angiography
image.)
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The problem of underestimation of stenoses in color Doppler
images is known from conventional CCDS. Technical factors are
responsible for the fact that the residual lumen of a high-grade
stenosis is mostly too widely demonstrated: the color signal
shown on the monitor of the ultrasound apparatus is not a
direct representation of the detected flow but rather the
result of an extensive electronic image processing.2 As a
consequence of spatial processing, measured volumes in the proximity of
a rapidly perfused stenosis canal in which no flow is actually
measured can in fact appear to give rise to flow signals. Since this
function of the image processing may produce differing results with
different ultrasound systems, the findings of independent investigators
may vary. For our investigations we used the same type of
apparatus (Acuson 128 XP) as Steinke and coworkers, and
thus this argument cannot be used to explain the differing results. We
also used the same apparatus settings as recommended by
this group as far as the appropriate information was given.
The conclusion of Steinke et al that "PDI further improves the
assessment of ICA stenosis" cannot be confirmed, even when
our results are not taken into account. (1) There is only a moderate
correlation between PDI and angiography findings,1 whereas
good correlations have been demonstrated previously in several studies
on comparison of Doppler sonography findings with those of
angiography3 4 5 or endarterectomy
specimens.6 7 (2) The PDI findings are not validated:
angiography was held to be responsible for the only modest correlation
between PDI and angiography. What then can be taken as the reference
standard? (3) The local degree of stenosis was determined
sonographically according to the European Carotid Surgery Trial, thus
only those angiographic methods—not an amalgamation of various
procedures—may be taken for comparison. (4) Sonographic longitudinal
images are in principle poorly reproducible because they are not
obtained under standardized conditions. If the method is to be used in
spite of this, then first of all the reproducibility of the findings
and the agreement between different observers must be confirmed. (5)
The finding presented as an example by Steinke et al in their
Figure 1
confirms our reservations: according to PDI the degree of
stenosis amounts to 38%; according to Doppler criteria at
a peak flow velocity of 1.7 m/s (illustration of the same finding in
another article by Steinke and Hennerici8 ), a
stenosis of almost 70% exists.
In conclusion, the method of Steinke et al1 thus rather
adds to the already existing "chaos in methodology" for measuring
carotid stenosis.9 CCDS is—also according to our
experience with a large number of cases—currently able to replace
preoperative angiography in the majority of cases of carotid
stenosis; however, this is possible only through use of
validated criteria for stenosis.
References
-
Steinke W, Ries S, Artemis N, Schwartz A,
Hennerici M. Power Doppler imaging of carotid artery
stenosis: comparison with color Doppler flow imaging and
angiography. Stroke.. 1997;28:1981–1987.[Abstract/Free Full Text]
-
Arning C. Farbkodierte Duplexsonographie der
hirnversorgenden Arterien. Ein Text-Bild-Atlas der methodischen
Grundlagen, normalen und pathologischen Befunde. Stuttgart, Germany:
Thieme; 1996.
-
Hennerici M, Aulich A, Sandmann W, Freund HJ.
Incidence of asymptomatic extracranial arterial
disease. Stroke.. 1981;12:750–758.[Abstract/Free Full Text]
-
Hunink MG, Polak JF, Barlan MM, O'Leary DH. Detection
and quantification of carotid artery stenosis: efficacy of
various Doppler velocity parameters. AJR Am
J Roentgenol.. 1993;160:619–625.[Abstract/Free Full Text]
-
Alexandrov AV, Brodie DS, McLean A, Hamilton P, Murphy
J, Burns PN. Correlation of peak systolic velocity and
angiographic measurement of carotid stenosis revisited.
Stroke.. 1997;28:339–342.[Abstract/Free Full Text]
-
Widder B, Friedrich JM, Paulat K, Hamann H,
Hutschenreiter S, Kreutzer C, Ott F, Arlart IP. Bestimmung des
Stenosierungsgrades bei Karotisstenosen: Ultraschall und iv DSA im
Vergleich zum Operationsbefund. Ultraschall Med.. 1987;8:82–86.[Medline]
[Order article via Infotrieve]
-
Suwanwela N, Can U, Furie KL, Southern JF, Macdonald
NR, Ogilvy CS, Hansen CJ, Buonanno FS, Abbott WM, Koroshetz WJ, Kistler
JP. Carotid Doppler ultrasound criteria for internal carotid artery
stenosis based on residual lumen diameter calculated from en
bloc carotid endarterectomy specimens.
Stroke.. 1996;27:1965–1969.[Abstract/Free Full Text]
-
Steinke W, Hennerici M. Neue Ultraschallverfahren.
TW Neurologie Psychiatrie.. 1996;10:736–742.
-
Toole JF, Castaldo JE. Accurate measurement of carotid
stenosis: chaos in methodology. J Neuroimaging.. 1994;4:222–230.[Medline]
[Order article via Infotrieve]
Response
Wolfgang Steinke, MD
Department of Neurology,
Marien-Hospital,
Düsseldorf, Germany
Michael Hennerici, MD;
Stefan Ries, MD;
Andreas Schwartz, MD
Department of Neurology,
University Hospital Mannheim,
Mannheim, Germany
Nikos Artemis, MD
Department of Neurology,
AHEPA Hospital,
Thessaloniki, Greece
We appreciate the interest of Dr Arning in the results of our
recent study.1 In an attempt to reevaluate the
diagnostic significance of PDI, he has compared this
technique to "usual Doppler criteria" in a sample of selected
cases but without angiographic confirmation and found its utility less
convincing than concluded in our study. Unfortunately, the Doppler
criteria used by Arning and coworkers have not been published in an
original scientific paper but refer to personal experiences of the
author of the letter published in a German textbook.2 The
criteria also do not correspond with those recently proposed by a large
board of experts in an international consensus meeting.3
Without vigorous testing and exactly defined criteria, however, any
classification of the degree of carotid stenosis based on
different criteria such as hemodynamics (ie,
Doppler) and morphology (ie, duplex ultrasound and angiography) is
misleading; eg, a peak systolic velocity of 170 cm/s alone
cannot simply be considered a valid criterion for a 70%
stenosis.
In contrast to our study, Arning does not provide systematic data
but instead argues from a small collection of cases with casual
angiograms, such as those illustrated in the Figure. Although he claims
to use the same instrumental setting as we did in our study, it is
obvious from the illustration that the gain of power for Doppler
color signals was inadequately adjusted, leading to overestimation of
the intrastenotic lumen diameter and to underestimation of the
degree of stenosis, respectively. The observation of a somewhat
lower rate of adequate visualization of the intrastenotic lumen
(85% in 40 ICA stenoses versus 92% in 128 stenoses in
our trial) and the reported difficulty in displaying high-grade
stenosis may simply reflect selection bias and problems in
technology. Because corresponding data from both velocity and amplitude
modes are not reported, the direct comparison with our study is
impossible, and the discussion missed the crucial difference made
between measurement of the local degree of stenosis in
sonography and angiography. The latter uses an approximation of the
distance between the vessel walls whereas the former directly images
wall and plaque texture.4 This was demonstrated by the
inclusion of different reference methods to validate PDI results in our
trial and should not be misinterpreted as "amalgamation of various
procedures."
Arning is correct when he states that the correlation of PDI and
angiography was only moderate. However, angiographic overestimation
using the local degree of stenosis (European Carotid Surgery
Trial) and underestimation using the distal degree of stenosis
(North American Symptomatic Carotid
Endarterectomy Trial) were not accounted
for.5 However, if angiography is referenced and the
procedure of measurement defined, PDI studies correlate very closely
with angiographic images. Different ultrasound methodologies must also
be defined and carefully used for classification purposes: procedures
imaging structural or flow conditions (2-D and 3-D tests) should not be
mixed with Doppler recordings from selected sample volumes
(1-D). Such inappropriate dimensional comparisons have repeatedly shown
a wide variability of results and have often contributed to the common
view of ultrasound as a subjective procedure with poor reproducibility.
"Homemade" Doppler criteria actually bear a great
responsibility in supporting this poor reputation, whereas
appropriately used objective imaging ultrasound procedures are a strong
shield in the battle against methodological chaos.
References
-
Steinke W, Ries S, Artemis N, Schwartz A,
Hennerici M. Power Doppler imaging of carotid artery
stenosis: comparison with color Doppler flow imaging and
angiography. Stroke.. 1997;28:1981–1987.
-
Arning C. Farbkodierte Duplexsonographie der
hirnversorgenden Arterien. Ein Text-Bild-Atlas der methodischen
Grundlagen, normalen und pathologischen Befunde. Stuttgarf, Germany:
Thieme; 1996.
-
de Bray JM, Glatt B. Quantification of
atheromatous stenosis in the extracranial
internal carotid artery. Cerebrovasc Dis.. 1995;5:414–426.
-
de Bray JM, Baud JM, Dauzat M, on behalf of the
Consensus Conference. Consensus concerning the morphology and the risk
of carotid plaques. Cerebrovasc Dis.. 1997;7:289–296.
-
Donnan GA, Davis SM, Chambers BR, Gates PC. Surgery
for prevention of stroke. Lancet.. 1998;351:1372–1373.[Medline]
[Order article via Infotrieve]
